Review residuals #3/#4/#6/#10: per-group in-process promotion lock (concurrent
route+auto promotion could double-create PRs), finalized-but-PR-less groups can
be repaired by re-promotion without re-merging, auto-promotion failures emit
merge:branch-group-promotion-failed instead of silent swallow, exported
reconcileBranchGroupPr for out-of-band merged reconciliation, and the merger
sync block drops its (store as any) casts (TaskStore already carries the
methods).
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